1. Field of the Invention
The present invention relates to the field of image scanning and, more particularly, to rotary scanners having rotating drums on which documents to be scanned are mounted.
2. Description of the Prior Art
Rotary drum scanners have been used for many years for image scanning due to the high resolution and high quality which they attain. These scanners were traditionally of two types, reflectance scanners and transmittance scanners. In both types, a document bearing an image to be scanned is mounted on a drum which rotates in the scanner, while an optical head is directed at the document to collect light and direct it to an optics box which converts it into a signal stream. While the drum rotates, it is gradually moved parallel to its rotational axis. The signal output by the optics box is thus representative of a line-by-line raster scan of the document.
Depending on the type of scanning (i.e. reflectance or transmittance), a light source is located either outside the drum, or within it. With a transmittance type scanner, the document being scanned is on a transparency, and the drum is translucent such that a light source from within the body of the drum transmits light through the surface of the drum and through the transparency being scanned. The light source is directed at the scanning head, and remains fixed in position relative thereto as the drum moves along its axial path.
In a reflectance type scanner, the light source is usually located outside of the drum, adjacent to or integral with the scanning head. The document being scanned is not a transparency, but is opaque and reflects light from the light source. Some of the reflected light reaches the scanning head, which collects it and directs it to the optics box. The drum need not be transparent for the reflectance type of scanner but, of course, it may very well be so. In both types of rotary scanners, the output scanning signal of the optics box may be analyzed, stored and/or used to assemble the final desired image.
Some modern scanners accommodate both reflectance and transmittance type of scanning using a translucent drum and light sources located both within the drum and outside of it. A scanning head outside of the drum receives image data either reflected from the surface of the scanned page or transmitted through the page, depending on whether it is operating in reflectance mode or transmittance mode.
More recently, rotary drum scanners have been designed to support drums of more that one diameter. Drum size is an important factor in scanner versatility. It dictates the available imaging area which, in turn, defines the maximum size original that can be scanned. Users that must scan large originals have no alternative to buying a scanner that will accommodate their maximum size original. However, at high resolutions, there is an inverse relationship between drum size and scanner throughput. The larger the drum, the longer the scan time. Consequently, having two drum sizes, one for large originals and one for higher throughput, provides greater flexibility. While the versatility of having two different drum diameters with the same scanner has proved popular, the actual support and control of the different diameter drums amounts to a complicated task. A precise and stable support mechanism is required which can support and drive each of the drums and which allows the interchange of drums with a minimum of effort.
Another problem specific to dual drum scanners involves the positioning of the light source and detection elements. Whether a scanner is of a transmittance type or a reflectance type (or accommodates both types), the position of the scanning head and the illuminating mechanism (whether reflectance or transmittance) must be adjusted to accommodate the particular drum being used.